The present invention generally relates to systems for rearing aquatic life and, in particular, to a method and apparatus for providing aerated water and nutrients to living organisms such as fish within a tank and for removing waste from the tank.
A substantial number of systems have been developed to enable fish farming or fish husbandry operations to be successfully conducted in a commercial setting. In some systems, a cage is immersed in a body of water, such as an ocean or lake, and fish are deposited within the cage until they have grown to the desired size. While immersed cages provide a nearly natural setting to the fish raised within and accordingly entail relatively few expenses for pumping and filtering water, the cages are subject to the vagaries of nature, such as storms or predators which may damage the cage and allow the fish to escape and water temperature changes which may adversely affect the feeding habits of the fish, and in the extreme, may cause death.
In other systems, fish farming is conducted on land in a large tank or tanks which have been filled with fresh or salt water. Fish eggs or fishlings are introduced to the tanks of water and are provided with nutrients to enable the growth of the fish to a commercially usable size. Tank systems are generally less susceptible than cage systems to storms and predators. Additionally, the water in the tank can be heated or cooled to maintain the water at or near a desired temperature.
For a tank type fish farming operation to be commercially economical, it is often necessary that a relatively high density of fish be placed in the water. Such a high density of fish rapidly consume the oxygen and nutrients available in the water and pollute the water with their metbolite waste products. If additional nutrients and oxygen are not provided to the water or if the waste products are not neutralized or removed, the water will no longer support the life of the fish being raised therein.
To provide the required oxygen and to remove the waste products, it is generally known to introduce freshly aerated water into the tank holding the fish and to remove the water which has been polluted by the waste products. It is common that such aerated water be pumped into the tank to provide a circulation of the aerated water throughout the tank.
To reduce the amount of water required in the operation of a fish farm, it is known to filter the waste filled-water, to aerate the filtered water, and to re-introduce the aerated, filtered water back into the tank. Such a closed system in which most of the waste water is reused reduces considerably the amount of water consumed in the fish farming operation.
A number of methods are known to aerate the water provided to a tank. For example, it is known to allow the water to set in a tank exposed to fresh air to permit the partial pressure of oxygen in the atmosphere to come to equilization within the partial pressure of oxygen in the water. To speed the aeration process it is known to allow the water to fall through the air, and/or over a trickling surface, to increase the mixing of oxygen with the water. It is also known to supply the water to the open top of a vertical tube within a concentric cylinder having its bottom end sealed. As the water is supplied to the tube, a supply of air is passed through the water and becomes entrained therein. As the water with the entrained air is forced to the bottom of the tube by the pressure of the water entering the tube, the water becomes supersaturated with the oxygen in the air.
While the U-tube efficiently oxygenates the water, the partial pressure of nitrogen in the air is also simultaneously applied to the water, resulting in a supersaturated solution of nitrogen in the water. Such a nitrogen rich solution cannot be withstood by the fish and, accordingly, measures must be taken to reduce the nitrogen concentration in the water.
Conventional pumps are used to move the water through the filtration system and aeration system and to provide momentum to the water being introduced to the tank, which enables circulation of the aerated water throughout the tank. The cost of continually operating such pumps often represents a considerable percentage of the cost of a fish farming operation. Consequently, methods and apparatus of fish farming which reduce the amount of water which must be pumped and/or the velocity at which the water is introduced into the tank are often very beneficial to a fish farming operation.
At least two types of tanks and associated aeration systems have generally been utilized in fish farming operations: rectangular tanks and cylindrical tanks. In many rectangular tanks, aerated water is provided to the fish in the tank by placing a series of intake nozzles along one end of the tank and a set of drains along the opposite end of the tank. The water exiting the intake nozzles is directed parallel to the longitudinal axis of the rectangular tank, i.e., in parallel lines aimed directly toward the drains.
The waste products of the fish include solid particles or offal components. In order to self-clean the tanks of the solids, i.e., to use the water to carry the solids to the drainage end of the tank, it is often necessary to introduce the intake water at a high volume and velocity. Conventionally, rectangular systems require 100 gallons per minutes of aerated water per foot of width of the tank for each three feet in depth of the tank. For example, in a rectangular tank having a width of 32 feet and a 6-foot depth, it is often necessary to provide up to 6,000 gallons of water per minute. If water is introduced at a lesser rate, the tank will generally not self-clean, i.e., carry off solid waste particles, and will not provide an efficient environment for fish rearing.
The performance of rectangular tanks in fish farming operations is often hindered by the need to exhaust water from the intake nozzle at a high volume and velocity so that the water has a sufficient momentum to travel the length of the tank to the drain and clean the tank. Because the introduction of water with such high velocity into the tank agitates the water already in the tank in the area surrounding the introduction point, the end of the tank at which the water enters is often not occupied by fish which prefer less turbulent water. As the length of the tank increases and consequently the need for higher velocity of the incoming water is increased, more of the intake end of the tank may be unused by the fish.
As the water passes through the tanks, the oxygen available in the water is used by the fish, the water picks up metabolites, including ammonia, thrown off by the fish. Accordingly, when the water reaches the vicinity of the drains, it may be so depleted of available oxygen and/or so polluted by the metabolites that fish will not habitate in the drain end of the tank. Typically, in such rectangular tanks, one-third to one-fourth of the drainage end of the tank is unusable because of the lack of oxygen and the high concentration of waste products in the water. Consequently, even if a rectangular tank is lengthened, the water area available for rearing of fish is not necessarily increased unless other measures are taken to increase the water flow rate.
As the water is drained from rectangular tanks, it may be filtered, aerated and pumped back to the intake nozzle of the tank for reintroduction into the tank or it may be supplied to a second, similar tank cascaded next to the first tank for introduction into the second tank. If the tanks are cascaded, the waste water from the final tank in the cascade may be discarded or filtered again and returned to the first tank in the cascade. Between the tanks of the cascade, aeration may be accomplished by allowing the water to fall to a lower level at which the next tank in the cascade is positioned.
Cylindrical tanks are also well-known for use in fish husbandry. Such tanks are often aerated by providing a series of intake ports along a radius of the tank, with all of the ports exhausting water in a direction perpendicular to the radius to establish a circular flow around the tank. Drains may be placed at the bottom of the tank near the center or approximately vertically below the radial intake ports. In this configuration of tank, water may be pumped through the intake ports, traverse the circular course of the tank and exit through the drains.
As noted above, in connection with rectangular tanks, for efficiency it may be desirable to filter the waste products from the water in the tank, aerate the water, and reintroduce the aerated, filtered water to the tank. Also, in a similar fashion to rectangular tanks, it is known to cascade a series of cylindrical tanks, so that the drained water from one tank may be aerated through a waterfall and applied to the intake ports of the next tank in the cascade to lower the requirement for pumping water.
An alternative aeration structure used in conventional cylindrical tanks provides a set of intake ports across an entire diameter of the tank with the nozzles of the intake ports pointed perpendicular to the diameter in the direction to promote a circular flow of the fluid within the tank.
An alternative to the longitudinally aerated rectangular tank and the radially aerated cylindrical tank is disclosed in the Ruckl U.S. Pat. No. 833,418, issued Oct. 16, 1906, in which intake water is introduced along an end of a rectangular tank which has fillets in each corner to help to circularize the flow of water within the tank.
While the operating cost of circular tanks used in fish rearing is generally 25% less than similarly capable rectangular tanks, cylindrical tanks are generally less efficient than rectangular tanks in the use of space occupied by the tanks. For example, if a fish farmer desires to add additional tanks to increase his production, rectangular tanks may be placed end-to-end and side-to-side, utilizing nearly all the available floor (or ground) space in a farming site. In contrast, cylindrical tanks when placed side-to-side abut only at a single point and, consequently, a considerable amount of floor space is not used in fish farms having a number of cylindrical fish tanks.
As an alternative to increasing the number of tanks to provide additional farming capability, a farmer could increase the size of the tanks. However, increases beyond certain sizes are not practical due to the high velocity of water which must be introduced into the tank in order to provide self-cleaning. Such high velocity water may result in turbulence in the tanks and result in unused space and in space shich cannot be used to house different sizes of fish.
It is, accordingly, an object of the present invention to provide a novel method and apparatus for providing aerated water to a fish tank.
It is a further object of the present invention to provide a novel method and apparatus for increasing the effective and usable size of tanks for the rearing of aquatic animals.
It is still a further object of the present invention to provide a novel method and apparatus to provide a fish tank with freshly aerated water with a reduced consumption of water.
It is yet another object of the present invention to provide a novel method and apparatus for the energy efficient circulation of water in a fish tank.
It is still another object of the present invention to provide a water management system for an energy efficient fish rearing tank.
It is still a further object of the present invention to provide a novel method and apparatus for aerating water for use in a fish tank with a reduced increase in nitrogen concentration.
These and other objects and advantages will be readily apparent to one skilled in the art for which the invention pertains from reading of the following detailed description when read in conjunction with the appended drawings.